1. Field of the Invention
The present invention relates generally to an improved interbody (for placement at least in part between adjacent vertebral bodies) threaded spinal fusion implant for the immobilization of vertebrae. The present invention is directed to threaded implants only and not to non-threaded implants. In particular, the invention relates to a threaded spinal fusion implant that is selectively directionally expandable and which specifically has height raising capabilities that are utilized once the implant is initially positioned. More particularly, the invention relates to a threaded implant having arcuate portions of upper and lower members that in a first, collapsed, or insertion position are angled to one another and form at least a portion of a frusto-conical shape along a substantial portion of the length of the implant.
2. Description of the Related Art
Threaded spinal fusion implants are known in the related art. The first artificial threaded spinal fusion implant was invented by Michelson and is disclosed in U.S. Pat. No. 5,015,247, which is hereby incorporated by reference.
Lordotic, frusto-conical, or tapered, threaded spinal fusion implants are also known in the art. By way of example, Michelson has invented such implants as disclosed in U.S. application Ser. No. 08/480,904, which is hereby incorporated by reference.
Expandable fusion implants are known in the related art. The first expandable spinal fusion (allowing for the growth of bone from vertebral body to vertebral body through the implant) implant was invented by Michelson and is disclosed in U.S. Pat. No. 5,776,199, which is hereby incorporated by reference.
Lordotic, frusto-conical, or tapered, spinal fusion implants have the advantage of restoring or enhancing spinal lordosis. Threaded spinal fusion implants offer the advantage of being easily positioned in the implantation space and of having excellent fastening or holding features. Expandable fusion implants offer the advantage of allowing for the placement of a potentially larger implant through a smaller opening in a patient""s body. Selective expansion along a single direction, (e.g. vertical only when correctly installed) offers the advantage of increasing the height of the implant and therefore the distraction of the disc space, but without a concomitant increase in the width of the implant.
There exists a need for an artificial interbody spinal fusion implant providing for all of the aforementioned advantages in combination.
In accordance with the present invention, as embodied and broadly described herein, there is provided an expandable threaded artificial interbody spinal fusion implant, having a shape that is generally frusto-conical when inserted, for insertion across a disc space between two adjacent vertebral bodies of a human spine. The threaded implant of the present invention includes an upper member having an arcuate portion adapted for placement toward and at least in part within one of the adjacent vertebral bodies and a lower member having an arcuate portion adapted for placement toward and at least in part within the other of the adjacent vertebral bodies. The arcuate portions of the upper and lower members have at least one opening in communication with one another for permitting for the growth of bone from a vertebral body to an adjacent vertebral body through the implant. The upper and lower members are articulated therebetween, preferably proximate one of the proximal ends and the distal ends of the upper and lower members and preferably allow for divergence between the articulating members at the end opposite the articulating end of the implant. The upper and lower members have a first position relative to one another that allows for a collapsed implant height and a second position relative to one another that allows for an increased height. The arcuate portions of the upper and lower members in the first position of the present invention are angled to one another and form at least a portion of a frusto-conical shape along the length of the implant. A portion of at least one helical thread is on the exterior of each of the opposed arcuate portions of the upper and lower members for engaging the adjacent vertebral bodies. The upper and lower members have a leading or distal end, an opposite trailing or proximal end, and a length therebetween. A blocker that is preferably in the form of an expander is located proximate at least one of the ends for holding at least a portion of the upper and lower members apart so as to maintain the increased height of the implant and resist the collapse of the implant to the collapsed implant height. Expansion of the implant preferably increases the implant height only, that is in a plane passing through the mid-longitudinal axis of the implant and the upper and lower members.
The blocker need not be in contact with the upper and lower members when the implant is initially inserted into the implantation space. The blocker may be a block or any type of spacer that is inserted between or otherwise holds apart the articulated upper and lower members after the implant is positioned so as to hold portions of the upper and lower members spaced apart the optimal height and angulation relative to one another. That is, the implant may be expanded with an extrinsic tool and then the expanded portions held apart in the second position by a third body blocker or blockers placed therebetween. Further, a physician may be able to select from a series of blockers having different heights usable with the same implant. The present invention includes expanding the implant with a tool, such as a spreader or a distractor, but is not limited to a scissors type, a rack and gear type, a threaded member type or any other type of particular external expander tool mechanism. Each tool nevertheless preferably engages the upper and the lower implant members to urge the implant apart. Then the blocker may be inserted into contact with the upper and lower members to maintain the implant at an expanded height. The height of the gap created by expanding the implant may be measured so that the appropriately sized blocker or expander may be inserted into contact with the upper and lower members depending upon the amount of distraction of the implant desired by the physician.
In a preferred embodiment, the blocker is in contact with the upper and lower members prior to the implant expansion, and the blocker is itself the expander, which may be operated by an extrinsic tool. By way of example only, the expander may rotate: to increase the height of the implant; in a single direction; more than 40 degrees and less than 140 degrees and more preferably approximately 90 degrees to move from a first insertion position to a second/deployed position; and in a plane perpendicular to the longitudinal axis of the implant to increase the height of the implant. The expander preferably remains in the same perpendicular plane relative to the longitudinal axis of the implant when rotated. In another embodiment the expander may be a member, such as a plate, a rod, or of any other configuration suitable for the intended purpose initially within the interior between the upper and lower members in a collapsed position that is erected to a more erect position when the implant is in the expanded position. The expander can be hollow or solid.
In a preferred embodiment, the expander preferably has means including, but not limited to, an opening, a projection, or a detent adapted to cooperatively engage a tool used to rotate the expander to increase the height of the implant. The opening, projection, or detent is adapted to cooperatively engage a tool that preferably rotates about an axis parallel to the longitudinal axis the implant to rotate the expander to increase the height of the implant. Rather then having an opening, a projection, a detent, or a central aperture, the expander may have two or more recesses or holes placed on or through the proximal face to engage a tool. In an alternative embodiment of the expander, cutouts may be positioned along a portion of the perimeter of the expander.
The expander is preferably located proximate at least one of the proximal end or the distal end of the upper and lower members. The expander, however, need not be so located. The expander may be spaced away from the end and even permit a hollow portion to exist on both the proximate and distal sides of the expander. The upper and lower members preferably have an interior surface therebetween and a hollow defined therein with the expander located proximate one of the longitudinal ends of that interior hollow. The hollow between the ends of the upper and lower members is preferably unobstructed by the expander so as to permit growth of bone directly through the hollow unobstructed by the expander from vertebral body to vertebral body through the implant transverse to the longitudinal axis of the implant. The implant may comprise a second and lesser hollow extending at least in part from the expander to the end of the upper and lower members proximate that expander. A preferred expander mechanism includes an expander in combination with cooperating surfaces of the end wall of the implant that guide and support the expander.
Preferred forms of interbody spinal fusion implants have a substantial hollow portion. Certain expandable interbody spinal fusion implants that increase in height only of the related art contain an expansion mechanism passing longitudinally therethrough or an expansion mechanism that is configured for movement of the expansion mechanism from proximate one end of the hollow portion to proximate the other end of the hollow portion, thus requiring the expander to pass through the length of the hollow portion. A preferred embodiment of the present invention overcomes these limitations.
The expander moves the arcuate portions of the upper and lower members from an angled orientation to a parallel orientation relative to one another; from a first angled orientation to a second angled orientation relative to one another; or from a first height at each end to a second and greater height at at least one and possibly both ends, but in each event the arcuate portions of the upper and lower members in the first or insertion position are angled to one another over a substantial portion of the length of the implant, and/or form at least a portion of a frusto-conical shape along the length of the implant. Each of the upper and lower members may structurally cooperate with a blocker, or expander so as to keep it located so as to function for its intended purpose. By way of example, each of the upper and lower members preferably has a track within which the blocker may be captured or the expander rotated. The tracks may be configured to permit the expander to rotate therein and then to move from side to side therewithin. The track of the upper member and the track of the lower member are preferably in the same plane and the plane is preferably perpendicular to the longitudinal axis of the implant.
A preferred expander has a first height in a first or insertion position and a greater second height when rotated or positioned into a second or deployed position to increase the maximum height of the implant from a first maximum height to a second maximum height. By way of example, at least one of the tracks of the upper and lower members preferably has a cooperating surface and the expander has a corresponding cooperating surface that contacts the cooperating surface of the track to orient the expander in a predetermined position. The cooperating surfaces preferably orient the expander within the implant such that the axis of rotation of the expander is parallel with the longitudinal axis of the implant and, more preferably, center the expander within the implant such that the axis of rotation of the expander coincides with the longitudinal axis of the implant. As may be advantageous for the further loading of the implant with fusion-promoting material, the expander may cooperate with the tracking surfaces of the upper and lower members to allow the expander to slide from side-to-side for easier access to the implant interior.
The implant is preferably packed full of bone or other fusion-promoting substances prior to expansion of the implant. Expansion of the implant results in a space being formed in the implant interior into which additional fusion promoting substances such as bone may preferably be packed. Rotating the expander within the implant causes a void that can be filled with bone. If the expander is configured to permit side-to-side movement, then packing of additional bone into the implant is easy.
When installing a preferred implant from the posterior approach to the spine, the implant is driven from the trailing end and the expander is at the leading end at the anterior aspect of the spine. When expanded, the implant installed from the posterior aspect leaves a void at the leading end of the implant near the anterior aspect of the spine because the leading end of the implant has been made taller, the void is preferably packed with bone. Additionally, the path left behind in the bone filled interior of the implant by the tool used to access the expander through the bone filled interior to position the expander is preferably packed with bone as well.
In a preferred embodiment of the present invention, the expander height change from the first position to the second position corresponds to substantially the same change in height of the implant along at least a portion of the length of the implant. The expander may be configured in different ways. A preferred configuration for a rotational expander includes: a first dimension corresponding to the width of the expander when the implant is initially inserted into the spine and to the height of the rotational expander when the rotational expander is rotated to increase the height of the implant; and a second dimension corresponding to the height of the expander when the implant is initially inserted into the spine and to the width of the expander when the expander is rotated to increase the height of the implant. The first dimension preferably is greater than the second dimension.
The expander may have an upper surface, a lower surface, and side surfaces as defined when the expander is positioned after rotation to increase the height of the implant. As used herein, the term xe2x80x9cside surfacesxe2x80x9d refers to those portions of the expander that extend from the upper member to the lower member after the expander has been rotated into its second or deployed position to increase the height of the implant. The xe2x80x9cupperxe2x80x9d and xe2x80x9clowerxe2x80x9d expander surfaces refer to those portions of the expander that are in contact with the upper and lower members when the implant is in its second or expanded configuration. Each of the upper and lower surfaces of the expander may lie generally in a plane and may be generally parallel to one another. The side surfaces and the upper and lower surfaces may be oriented so as to substantially form a parallelogram, which will typically be in the shape of a rectangle generally.
A preferred expander is in the form of a modified rectangle or rhomboid. The expander generally has a longer dimension and a shorter dimension. When the expander is in a first position, the short dimension spans the distance between the upper and lower members and when the expander is in the second position, the expander""s long dimension spans the distance between the upper and lower members.
The expander may have a cross-section with the side surfaces intersecting the upper and the lower surfaces at junctions, which may be two diametrically opposed corners and two diametrically opposed arcs. The two diametrically opposed arcs may be each of the same radius and, preferably, the diagonal or modified hypotenuse xe2x80x9cMHxe2x80x9d between the opposed arcs has a maximum dimension that generally approximates the distance between the upper and lower surfaces such that, when the expander is rotated from a first insertion position toward a second/deployed position, no substantial over-distraction occurs between the adjacent vertebral bodies as would occur if the height of the implant was increased markedly beyond that obtained in the second/deployed position. The two diametrically opposed corners may form a 90-degree angle. The expander preferably has a fixed shape during movement from a first insertion position to a second/deployed position within the implant.
In a preferred embodiment, a modified hypotenuse or diagonal xe2x80x9cMHxe2x80x9d is the dimension between the two diametrically opposed arcs that allows for the rotation of the expander from a first position to a second position without substantial over-distraction occurring during this process. The phrase xe2x80x9cwithout substantial over-distractionxe2x80x9d is defined as distracting the vertebral bodies in the range of elastic deformation and short of plastic deformation and tissue failure. To avoid any ambiguity regarding the phrase xe2x80x9cwithout over-distraction,xe2x80x9d this phrase and the individual words contained therein are not being used as they may be in their normal or ordinary use, but are being used as defined in this application only. In the example of this rotational expander, the MH could be identical in length to the height thereby assuring literally no overdistraction. It may be preferred, however, to have the MH just slightly greater in length than the height to insure the stability of the expander in the expanded or second position because this would then require additional force over the stable position to derotate the expander.
In accordance with an embodiment of the present invention, a second expander may be located between the upper and lower members for moving at least a portion of the upper and lower members away from one another to increase the height of the implant as defined by the maximum distance between the arcuate portions of the upper and lower members proximate that expander. All of the features described herein for the expander may also be applicable to the second expander. Additionally, the second expander may be located proximate an end of the implant opposite the other expander, thereby providing an implant capable of being expanded at both ends of the implant. The increased height of the implant resulting from moving the two expanders may be constant or varied along the length of the implant according to the desired configuration of the implant and the relative dimensions of the individual expanders. A given implant may be adapted to receive or cooperatively engage a series of progressively sized (taller) blockers or expanders to allow the surgeon to make a final height selection at the time of surgery.
In accordance with an embodiment of the present invention, the implant may include an expansion mechanism including the expander and at least one partial wall structure preferably located proximate an implant end that guides and holds the expander in a predetermined position.
The implant may have an overlapping step-cut wall junction between the upper and lower members, which offers as some of its advantages: increasing the lateral rigidity of the implant, holding the implant in the closed first position until expanded, and to the extent desired retaining the fusion-promoting materials within the implant. The wall junction may be either solid or perforated.
One of the upper and lower members preferably has an interior wall extending toward the other of the upper and lower members and, more preferably, has two interior walls extending from each side of the arcuate portion. The interior walls may be aligned parallel with the longitudinal axis of the implant. The other one of the upper and lower members preferably has an interior-contacting surface adapted to contact or receive the interior longitudinal wall.
By way of example, one of the upper and lower members may have a longitudinally extending interior wall, which is preferably unexposed, extending toward the other of the upper and lower members when the implant is in an initial insertion position. When the implant is in the final expanded or deployed position the implant has a preferred shape such that each of the arcuate portions of the upper and lower members are separated by at least a portion of interior wall, which in this position preferably has an exposed side.
The upper and lower members in certain embodiments are articulated to one another so one of the respective ends of the upper and lower members remain articulated while the other of the respective ends of the upper and lower members are free to move away from one another. In a preferred embodiment, the articulating means is achieved without a third member, such as an axle shaft, for example, passing through the implant. The articulating means preferably is formed into the implant walls themselves, and in a further preference in such a way that the two-implant halves may be articulated when at 90 degrees to each other. The halves then are moved, much like a book closing, toward each other prior to insertion into the implantation space in the spine. Once the upper and lower members are closed from the approximately 90 degrees articulating position, much like closing the leaves of a book, the upper and lower members of the implant are locked together at the articulation so that the members will not disarticulate when in use. Other types of articulation as would be known to one of ordinary skill in the art are within the scope of the present invention.
By way of example, the upper and lower members preferably have a cooperating rotational articulation or pivot point between a proximate one of the proximal end and the distal end of the upper and lower members. The cooperating rotational articulation preferably is proximate one of the proximal end and the distal end of the upper and lower members at an end opposite the expander when only one end is to be expanded. A preferred rotational articulation configuration includes cooperating brackets and projections configured such that articulation therebetween occurs when the upper and lower members are substantially perpendicular to one another. Such a configuration offers the advantage that the brackets and the projections will not disengage one another when articulated for use such as insertion into the spine and subsequent expansion within a range of movement of the upper and lower members resulting from the expander positioning.
When the implant is in the final or expanded position the implant may take the general form of a frusto-conical shape split along a horizontal plane through its mid-longitudinal axis wedged upper half from lower half by an inclined plane.
At least one and preferably both of the upper and lower members may have a screw hole passing through the trailing end, which preferably is adapted to receive a screw passing through the end of the upper and lower members and from the interior of the implant into each of the adjacent vertebral bodies to anchor the implant, further stabilize those vertebral bodies relative to each other, prevent undesirable motion at the vertebral body implant interfaces, increase the compressive load at the implant trailing end, prevent rocking and thereby to mitigate against excessive peak loads, and to more uniformly distribute loads imparted to the implant over the length of the implant to the adjacent vertebral bodies. The implant may have a side configured, when in the expanded position, to cooperate with another interbody spinal fusion implant so as to allow the pair of implants to have a reduced combined width.
The trailing end of the implant preferably has a tool-engaging portion, but the implant may be adapted to cooperatively engage a driver at another location or by any means as would be known to one of ordinary skill in the art. This tool-engaging portion is adapted to engage an insertion tool that holds and rotates the implant into position. The configuration of the tool-engaging portion may be an opening, and more particularly an opening that is along the longitudinal axis of the implant to facilitate the use of an insertion tool that rotates the implant into an inserted position. It is appreciated that the tool-engaging portion need not be an opening. A hole or a blind hole, threaded or otherwise, is preferred in another embodiment. In another preferred embodiment the opening preferably is a threaded slot that functions to cooperatively engage and disengage a tool for use in inserting the implant. In specific embodiments, the leading or trailing end may have wall portions, and/or be adapted to cooperatively engage a cap. Either the end wall portions or a cap may have an opening or openings, which may function to hold fusion-promoting materials within the implant and/or, permit vascular access and bone growth therethrough.
For an embodiment of an implant of the present invention having one expander, the main access opening is preferably at the end opposite from the expander. The main opening may be at either the distal or proximal end of the implant. The end of the upper and lower members containing the expander may serve as a secondary access opening.